1. Field of the Invention
This invention generally relates to a technique for applying corona ions to a charge receptor or imaging surface, and, in particular, to a corona charging device for charging an imaging surface such as the surface of a photoreceptor with corona ions.
2. Description of the Prior Art
In an electrophotographic machine, typically as shown in FIG. 1, there is provided a corona charging device 2 opposite to the surface of a charge receptor 1 such as a photoreceptor or photoconductive layer which is supported on an electrically conductive backing 8 which is grounded. The corona charging device 2 is generally comprised of a corona wire 3 and an electrically conductive shield 7 which partly encloses the corona wire 3 and is connected to ground. A d.c. high voltage is applied to the corona wire by a high voltage supply 3a. If charge receptor 1 is comprised of a photoconductive material, the surface of the charge receptor 1 is charged uniformly by corona ions applied by the corona charging device 2 in a dark place. Then, a light image is exposed to dissipate the charge selectively to form an electrostatic latent image, which is then developed into a visual toner image. Thereafter, the toner image is transferred to a transfer medium such as transfer paper. It is also true that a reproduced image may be obtained by using a charge receptor which is comprised of a material other than a photoconductive material. For example, the charge receptor 1 may be comprised of a dielectric material.
When such a corona charger is applied to electrophotographic machines such as electrophotographic copying machines, it is important that it can charge an imaging surface uniformly across its entire surface. If irregularities in charging are present, they could cause irregularities in density in resulting images thereby producing reproduced images of poor quality. In order to charge the imaging surface uniformly as well as stably, each of the following three factors contributing to the performance of corona charging must be uniform not only in function but also in properties.
(1) Corona charger. PA1 (2) Atmosphere ( usually air, including charge carriers ) between corona charger and charge receptor. PA1 (3) Charge receptor. PA1 (a) Charging of a charge receptor is to be carried out in a low humidity environment having charge carriers of low concentration. PA1 (b) A charge receptor has a large capacitance, requiring a large amount of charges to obtain a surface potential of desired level. PA1 (c) A charge receptor moves too slowly and thus fresh air is difficult to flow into the charging region.
It is to be noted particularly in electrophotographic machines that with respect to the above three factors, not only uniformity in space but also uniformity in time are required since the imaging surface is repetitively used. The charge carriers mentioned in item (2), for example, include H.sup.+ (H.sub.2 O).sub.n in the case of positive coronas.
It has been confirmed that non-uniformity in space and/or time is created to produce irregularities in the atmosphere, which result in non-uniform charging, under each of the following conditions.
Various proposals have been made to cope with the problems mentioned in items (1) and (3); however, no viable proposal has yet been made with respect to item (2). Thus there has been a need to develop a new device capable of charging a charge receptor uniformly as well as stably which is not influenced by the surrounding condition or atmosphere.